1. Field of the Invention
The present invention relates to a process for electroplating a metallic material with an iron-zinc alloy. More particularly, the present invention relates to a process for electroplating a metallic material, for example, a steel strip, in an electroplating sulfate liquid containing iron (Fe.sup.2+) and zinc (Zn.sup.2+) ions to form an electroplated iron-zinc alloy layer having a desired composition while preventing undesirable oxidation of ferrous ions in the electroplating sulfate liquid.
2. Description of the Prior Art
In usual electroplating procedures, it is known that a chemically noble metal deposits preferentially to a chemically base metal. This phenomenon is called normal type deposition. However, in an iron-zinc alloy electroplating procedure in which iron and zinc are contained mainly in the form of sulfates thereof in an electroplating liquid, zinc, which is a chemically base metal, deposits preferentially to iron, which is a chemically noble metal. This phenomenon is referred to as anomalous type deposition. Especially, where the electroplating liquid is contaminated with impurity ions, for example, Sn and Sb ions, the anomalous type deposition is promoted.
Due to the above-mentioned anomalous type deposition, and in order to obtain an electroplated iron-zinc alloy layer having a desired composition, it is necessary to make the molar ratio of zinc ions (Zn.sup.2+) to the sum of zinc ions and ferrous ions (Zn.sup.2+ +Fe.sup.2+) in the electroplating liquid significantly small in comparison with the molar ratio of zinc to the sum of zinc and iron in the resultant iron-zinc alloy having a desired composition. In this electroplating procedure, a fluctuation in the composition of the electroplating liquid, even if it is small, results in a large fluctuation in the composition of the resultant iron-zinc alloy. This phenomenon causes the control of the electroplating procedure to become difficult.
Accordingly, it has been strongly desired to provide a new iron-zinc alloy electroplating process in which the molar ratio of zinc ions to the sum of zinc and iron in the electroplating liquid is the same as or very close to that in the resultant electroplated iron-zinc alloy. In other words, it is desired to provide a new iron-zinc alloy electroplating liquid in which iron and zinc can deposit in a manner similar to or very close to the normal type deposition.
For example, Japanese Unexamined Patent Publication (Kokai) No. 57-192284 (1982) discloses an improved process for electroplating a steel strip with an iron-zinc alloy. In this process, an electroplating liquid containing 5 to 50 g/l of a polybasic carboxylic acid and 10 to 100 g/l of sodium acetate in addition to predetermined amounts of iron sulfate and zinc sulfate, is used.
It is known that when the iron-zinc alloy electroplating liquid is recycled in the electroplating procedure, an amount of air is introduced into the electroplating liquid. The introduced air serves to oxidize ferrous ions (Fe.sup.2+) in the electroplating liquid. Otherwise, when an insoluble anode is used, ferrous ions (Fe.sup.2+) are oxidized with oxygen generated on the anode or by a direct electrode reaction with the anode so as to produce ferric ions (Fe.sup.3+). The above-mentioned oxidation phenomenon results in an undesirable increase in concentration of ferric ions (Fe.sup.3+) in the electroplating liquid. The increased concentration of ferric ions (Fe.sup.3+) lowers the efficiency of the electroplating procedure and results in an unsatisfactory quality of the resultant electroplated alloy layer.
Accordingly, it has been strongly desired to provide a new iron-zinc alloy electroplating liquid in which the air oxidation and anodic oxidation of ferrous ions are strictly restricted. In the electroplating process, ferrous ions (Fe.sup.2+) and zinc ions (Zn.sup.2+) in the electroplating liquid are consumed and additional ferrous ions and zinc ions are fed from metallic iron and zinc which are capable of reducing the ferric ions. Accordingly, it is strongly desired to provide an electroplating liquid in which the reduction of ferric ions can be effected at a high efficiency.